To read/write information at a position on a spinning recording medium in a magnetic disk drive such as a hard disk drive, motion control is performed to move a magnetic head to the read/write position. If shock is given from external during this motion control, positioning of the magnetic head to the read/write position is difficult. Therefore, it is recently considered to configure such a control circuit that external shock is detected by a sensor and the signal from the sensor, after processed by a filter, is used to correct the magnetic head position control signal or remove the affection of the external shock.
FIG. 4 shows an example of a magnetic disk drive where such a circuit configuration is applied. It comprises a magnetic recording medium 1, a magnetic head actuator 2, a read/write circuit 3, a controller circuit 4, a driver circuit 5, a shock sensor 6 and a feed-forward circuit 7. These respective components are accommodated in and fixed to a base B.
On the magnetic recording medium 1, a plurality of concentrical tracks are formed. Data is written within these tracks.
The magnetic head actuator 2 is driven by a voice coil motor VCM to pivot, which moves the magnetic head to a read/write position on the magnetic recording medium 1.
The read/write circuit 3 generates a recording signal from recording data, which is entered from the controller circuit 4, and outputs it to the magnetic head. In addition, this read/write circuit 3 generates regeneration data from the regeneration signal, which is output from the magnetic head, and outputs it to the controller circuit 4.
The controller circuit 4 exchanges recording data and regeneration data with a host. In addition, this controller circuit 4 generates a position error signal (PES) indicative of the magnetic head's deviation from the read/write position by using the servo signal recorded on the magnetic disk medium 1 and, based on this position error signal (PES), generates a feedback control signal for controlling the magnetic head so as not to deviate from the track.
To move the magnetic head to an information read/write position on the magnetic recording medium 1 as instructed from the host, the controller circuit 4 generates a control signal based on the instruction and corrects this control signal by adding/subtracting the feedback control signal to/from the control signal to generate a primary corrected control signal.
Further, the controller circuit 4 generates a secondary corrected control signal by adding/subtracting the primary corrected control signal to/from a feed-forward signal which is output from the feed-forward circuit 7.
From the controller circuit 4, the driver circuit 5 receives the secondary corrected control signal to control the voice coil motor VCM. To the voice coil motor VCM, the driver circuit 5 outputs a control current whose amplitude is determined according to the secondary corrected control signal.
If a shock is given to the magnetic disk drive, the shock sensor 6 outputs a digital value indicating the magnitude of the shock. The feed-forward circuit 7 comprises a phase shift filter circuit 11, an adaptation circuit 12 and a feed-forward filter 13 as shown in FIG. 5.
Here, transfer function P is used to represent the voice coil motor VCM while transfer function Gc represents the controller circuit. Then, the response characteristic Gm from the output of the feed-forward filter 13 to the magnetic head actuator 2 is given as below.
                              G          m                =                  P                      1            +                                          G                c                            ⁢              P                                                          (                  Equation          ⁢                                          ⁢          8                )            
The phase shift filter circuit 11 receives the output signal of the shock sensor 6 as an input and shifts the phase of the output signal of the shock sensor 6 so that the phase of the output signal of the shock sensor 6 falls within a predetermined range (for example, ±π/2 radians) with respect to the phase of the signal when it has just passed the transfer function Gm given by Equation 8.
The adaptation circuit 12 receives the position error signal (PES) from the controller circuit 4 as an input and the output signal of the phase shift filter circuit 11 as an input. Based on the output signal of the phase shift filter circuit 11 and the position error signal (PES), this adaptation circuit 12 uses the LMS (Least Mean Square) algorithm to control the coefficients of the respective powers of z in the feed-forward filter 13 so as to make smaller the position error signal (PES).
The feed-forward filter 13 is a FIR (Finite Impulse Response) filter. The coefficients of the respective powers of z in its transfer function are controlled by the adaptation circuit 12. This feed-forward filter 13 filters the output signal of the shock sensor 6 and outputs it to the controller circuit 4 as a feed-forward signal.
In this conventional example, anti-shock dynamic feed-forward controlling of the amplitude of current supplied to the voice coil motor VCM can raise the shock resistance.
Defining Gbt as the shock transfer function from the base B to the magnetic head actuator 2, the transfer function Gff of the feed-forward filter 13 in the above-mentioned conventional magnetic disk drive should be set as below:
                              G          ff                =                  -                                    G              bt                                      G              s                                                          (                  Equation          ⁢                                          ⁢          9                )            
where the Gs is the output signal of the shock sensor 6. Generally, however, to realize this type of transfer function, the feed-forward filter 13 is required to be an IIR (Infinite Impulse Response) filter, not a FIR (Finite Impulse Response) filter. Generally, the IIR filter is not always stable. For the IIR filter to stably operate, its coefficients must be set carefully.
However, in the case of the technique mentioned above as a conventional example where the filter's coefficients are changed adaptively, the filter is configured as a FIR filter since it is not yet possible to secure the stability if the filter is configured as an IIR filter.